I was asked to do this by a couple of members, I had thought I had done something similar a while back but couldn't find anything digging through my old Fanposts, so if I had anyone chasing their tail looking, sorry about that.
The scout scale is a grading system teams use to identify the five tools; hitting, hitting with power, speed, running and throwing. Each tool is given a grade anywhere from 2 (or 20) to 8 (or 80). Some teams will use a half grade and some teams expand the scale to a 1-10, but for the most part the 2-8 is the consensus.
Scouts who primarily look at amateurs will use a two number system such as 3/4 or 5/7. The first number means current or today, the second number is where he is expected to be once his skills project out, which is called Overall Future Potential, or OFP. Pro scouts will often just use one, such as Joey Gallo's power is an 8, because he's already experienced pro competition which makes his OFP less speculative.
On the bottom end of the scale is the 2, or 20 if you prefer. That is considered less than pro quality; for a player to be signed with a 20 they must have a top of the scale 80 on the other side to balance out. For example, Adam Dunn is a 20 defender but has 80 power which balances out to a 50 between the two, which makes him an average ML player in those two areas. The compromise then is to DH him almost exclusively, which nullifies his 20 altogether.
A grade of 50 is considered major league average. I know some teams take the scale to the extreme and grade by position, which means a guy like Jacoby Ellsbury could have a 40 arm as a centerfielder but a 55 as a left fielder. That doesn't happen often but I have seen it and more than once, so it's out there. I think it has merit and use it myself when the situation requires, like when scouting the Arizona Fall League.
I've also seen grades in reverse, meaning something like a 60/50. In cases like this a scout may have used his own judgement based on the player's circumstance, then made an adjustment later after getting looks in a different environment. An example here would be Bryce Harper's power; he was an 80 when he was an amateur then downgraded after he signed to a 70 because he hadn't been graded using a wood bat. Again, doesn't happen often but I have seen it.
The two most common errors I see with the application of the scale are too large a gap between current and OFP, say a 25/50. What you're saying is a guy is a high school level hitter now but in three years could be major league average, which isn't going to happen. You could go 25/35 or 30/40, maybe even 3/5 as an extreme, but more than one number or a number and a half isn't feasible.
The other is "scouting with the eye". I know real scouts do this, but they do it for a living and have years and even decades of experience. Most people who use the scale don't have the knowledge of the tools to use it correctly; I've seen many comments here where people will grade a players hit tool just by looking at his stat line. Sorry, but no, it doesn't work that way.
Over the next couple of weeks I'll put up additional posts going over what scouts look for in the tools and talk about some common misconceptions such as the difference between makeup and character and what scouts actually use radar guns for.
Hope this helps for now, any questions comment me.